As semiconductor devices have become more highly integrated in recent years, circuit interconnections have become finer and a distance between these circuit interconnections have become smaller. In case of photolithography which can form interconnections that are at most 0.5 μm wide, it is required that surfaces on which pattern images are to be focused by a stepper should be as flat as possible because a depth of focus of an optical system is relatively small. A polishing apparatus for performing chemical mechanical polishing (CMP) has been used for planarizing a semiconductor wafer.
In a polishing apparatus for polishing and planarizing a surface of a semiconductor wafer on which a device pattern is formed, a non-woven fabric has heretofore been used as a polishing cloth attached to an upper surface of a polishing table. However, as ICs and LSIs have become more highly integrated in recent years, it is required to reduce differences in levels of a surface, to be polished, of a device pattern during polishing. In order to meet a demand for polishing so as to reduce differences in levels of a surface, to be polished, of a device pattern, a polishing cloth made of a hard material, e.g., a polishing cloth of foam polyurethane, has been used.
This type of polishing apparatus comprises, as shown in FIG. 21, a polishing table 402 having a polishing cloth (polishing pad) 400 attached thereon and constituting a polishing surface, and a top ring 404 for holding a substrate W as a workpiece to be polished, such as a semiconductor wafer, in such a manner that a surface to be polished faces the polishing cloth 400. A semiconductor wafer W is polished by this polishing apparatus as follows: The polishing table 402 and the top ring 404 are independently rotated, and, while a polishing liquid is supplied from a polishing liquid nozzle 406 provided above the polishing table 402, the semiconductor wafer W is pressed against the polishing cloth 400 on the polishing table 402 at a predetermined pressure by the top ring 404. For example, a suspension of fine polishing particles of silica or the like in an alkali solution is used as the polishing liquid supplied from the polishing liquid nozzle 406. Thus, the semiconductor wafer W is polished to a flat mirror finish by a combined effect of a chemical polishing effect attained by the alkali and a mechanical polishing effect attained by the polishing particles.
When the semiconductor wafer W is brought into contact with the polishing cloth 400 and the polishing table 402 is rotated to perform polishing, polishing particles or polishing wastes are attached to the polishing cloth 400, resulting in a change in properties of the polishing cloth 400 and a deterioration in polishing performance. Therefore, if an identical polishing cloth 400 is repeatedly used for polishing semiconductor wafers W, problems such as lowered polishing rate and uneven polishing are caused. In order to overcome such problems, conditioning called dressing is performed before, after or during polishing of a semiconductor wafer to regenerate a polishing cloth.
When dressing of a polishing cloth is performed, a dresser 408 is provided in the polishing apparatus, and polishing cloth 400 is dressed by the dresser 408 at a time of replacement of a semiconductor wafer W to be polished, for example. Specifically, while a dressing element attached to a lower surface of the dresser 408 is pressed against the polishing cloth 400 on the polishing table 402, the polishing table 402 and the dresser 408 are independently rotated to remove polishing particles and polishing wastes attached to a polishing surface of the polishing cloth and to flatten and dress the polishing surface in its entirety, whereby the polishing surface is regenerated.
With respect to dressing of a polishing cloth, in accordance with properties of the polishing cloth, the conventional polishing apparatus comprises one dresser selected from the group including a contact-type diamond dresser having diamond particles, a contact-type brush dresser having a brush, and a non-contact-type dresser for ejecting a fluid jet toward a surface of the polishing cloth to perform dressing.
However, during dressing of a polishing cloth, it has become necessary to use different dressers, for example, a dresser for thinly shaving a surface of the polishing cloth for initial surface conditioning before use during polishing, and a dresser for removing clogged aggregation of slurry (polishing liquid) and polishing wastes from the polishing cloth during a polishing process. If the clogged aggregation of the slurry (polishing liquid) or the polishing wastes are not removed from the polishing cloth, there is a high possibility that polishing particles and polishing wastes become attached to the polishing cloth to deteriorate a polishing function, or a surface, to be polished, of a semiconductor wafer is scratched, resulting in lowered yield. Therefore, in the conventional polishing apparatus, two or more dressers having different dressing elements should be replaced as needed. This work is troublesome and disadvantageously lowers throughput of polished semiconductor wafers.
Further, in the conventional polishing apparatus, if the top ring unit and the dressing unit are provided adjacent to each other for reasons of restriction of a layout, or if a plurality of top ring units and a dressing unit are arranged on a single table so as to interfere with each other, then dressing can be performed only when the top ring does not perform polishing and is positioned at a stand-by position. As a result, polishing cannot be performed during dressing, and hence a number of semiconductor wafers to be polished per unit time is decreased.
A thickness of a thin film formed on a surface of a semiconductor wafer varies from position to position in a radial direction of the semiconductor wafer depending on a film deposition method or characteristics of a film deposition apparatus. Specifically, the thin film has a film thickness distribution in the radial direction. Further, the film thickness distribution varies depending on a type of film deposition method performed or film deposition apparatus used. Specifically, a position and number of portions having a large film thickness in the radial direction, and difference in thickness between thin film portions and thick film portions, vary depending on the type of film deposition method performed or film deposition apparatus used.
However, with the aforementioned conventional dresser, an entire polishing surface is uniformly dressed, so that the entire polishing surface is regenerated to a uniform condition, whereby the entire polishing surface has uniform polishing performance. Therefore, if an entire area of a semiconductor wafer is uniformly pressed against the polishing surface after the polishing surface is regenerated by the conventional dresser, then a polishing rate is identical over the entire area of the semiconductor wafer. Thus, it is impossible to perform polishing in accordance with the above film thickness distribution, and hence a semiconductor substrate is excessively polished at portions having a small film thickness and is insufficiently polished at portions having a large film thickness.